Automatic control system for precision resistor manufacture



April 4, 1961 A. BLAUSTEIN 2,978,364

AUTOMATIC CONTROL SYSTEM FOR PRECISION RESISTOR MANUFACTURE Filed March5, 1956 2 Sheets-Sheet 1 4 AMPL/F/Eq INVENTOR: Aaron B/ausfe/n,

l8 BY ATTORNEY April 4, 1961 A. BLAUSTEIN 2,978,364

AUTOMATIC CONTROL SYSTEM FOR PRECISION RESISTOR MANUFACTURE Filed March5, 1956 2 Sheets-Sheet 2.

1 Aaron B/ausrein,

ATTORNEY INVENTOR United States Patent AUTOMATIC CONTROL SYSTEM FORPRECISION RESISTOR MANUFACTURE Aaron Blaustein, Hicksville, N.Y.,assignor to Fairchild Camera and Instrument Corporation, a corporationof Delaware Filed Mar. 5, 1956, Ser. No. 569,369

9 Claims. (Cl. 117-227) This invention pertains to control systems andmore particularly to a novel apparatus for measuring the resistance andcontrolling automatically the deposition of an electrical resistivecoating upon a support for insulating material.

In the manufacture of fixed and adjustable resistors, the resistivematerial is usually applied as a thin film upon a supporting base ofinsulating material by one or a combination of various methods includingprinting, vaporizing, cathode sputtering, and the like. The vacuumvaporization technique, while presenting many advantages from thestandpoint of electrical precision and noble qualities in the resistivematerial, involves difiiculty in that ordinarily the achieved resistanceof the deposit can only be determined by interrupting the deposition,and usually removing the element from the vacuum chamber. Even wherearrangements are provided for continuously monitoring the resistance, itis difficult to interrupt the vaporization process at precisely theright instant to yield the desired ultimate resistance in the finisheddeposit.

It is accordingly a principal object of the present invention to providea novel electronic system, which will automatically discontinue thedeposition process when the desired resistance value has been attained.As a subsidiary feature, this same electronic apparatus permitsallowance for the fact that discontinuance of vaporization may not occuruntil a short period after the vaporizing circuit has been opened. Thus,allowance may be made for the tendency of the coating apparatus tooverrun slightly when its control circuits are broken.

Another object of the invention is to provide apparatus of this type inwhich allowance may readily be made for the fact that the depositedresistance as measured under deposition conditions will not ordinarilybe equal to the value of said resistance after removal from the vacuumchamber; for example, due to the considerable change in temperaturebetween these two conditions.

It is another object of this invention to provide an apparatus forcontrolling the total deposition of conductive or resistive coatingsapplied to an insulating backing by thermal vaporization or likemethods.

Still another object of this invention is to provide a novel electronicsystem for automatically measuring the electrical resistance of acoating being applied to an insulating support, without interrupting thedeposition process.

Another object of this invention is to provide an electronic system forautomatically controlling the deposition of a resistive film upon aninsulating support and at the same time measuring the actual resistanceof the film as it is being deposited.

The use of this invention in a thermal vaporization apparatus fordepositing a resistive coating upon an insulating support willautomatically cut off evaporation action when the resistance filmreaches a desired predetermined value. This automatic control systemeiiec tively eliminates the human error caused by slow respouse andinaccurate judgment by the operator of the coating apparatus, thusresulting in increased production and greater accuracy of the product.

The above and other objects and advantages of this invention will bestbe understood from the following specification, taken in connection withthe accompanying drawings, in which:

Fig. 1 is a diagram, partly schematic, of the novel electronic controlsystem as utilized with a thermal vaporization type coating apparatus,

Fig. 2 is an exploded perspective view of the object being coated andthe control electrodes applied thereto, and

Fig. 3 is a circuit diagram of one embodiment of the control systemillustrated in Fig. l.

The novel apparatus now to be described is exemplified as used in anapparatus employed for fabricating rotary contact potentiometerresistance elements of small size and high precision. The methodsemployed, and the apparatus used for depositing the resistive coating,form no part of this invention and are described in detail in theconcurrently filed U.S. Patent application of Lawrence B. Krauss, SerialNo. 569,351, titled Making Precision Film Resistors. Although thisinvention will be described herein in conjunction with a thermalvaporization apparatus for depositing a thin layer of resistive materialupon an insulating support, it is understood that the invention canequally well be applied to other coating devices or methods where it isdesired continuously or intermittently to measure the conductivityorresistivity of the coating being applied, and also automatically tocontrol the deposition or rate of deposition of the coating beingapplied.

Referring to Fig. l of the drawings, there is illustrated a coatingapparatus of the thermal vaporization type in which the novel controlsystem comprising this invention is utilized. The coating apparatusincludes a base 10, preferably metallic, which forms a support uponwhich an air tight bell jar 12 of glasso-r the like is arranged to bedisposed in a well known manner, for evacuation of the bell jar bysuitable vacuum pumps. Suitable vacuum gages and like accessories, wellknown in the vacuum deposition art may be provided. For measurement,control and manipulation of the parts within the bell jar during thecoating operation, electrical leads are brought out through the base 10by means of suitable vacuum seals.

The object to be coated, designated by the reference numeral 14, is inthis example an insulating base element in the form of a ceramic orglass disc having a central aperture therein. Referring to Fig. 2 it isseen that the disc 14 has been prepared for this coating operation bythe application on the upper surface thereof of two separate contactareas 15 and 17. These contact areas are of a suitable conductivematerial such as silver; they are of egligible resistance and areconnected to the underside of the disc 14 by conductive strips 19 and 21respectively to make contact with the resistive coating to be applied tothe underside of the disc. It is noted that the conductive strips 19 and21 pass over the edge of the disc 14 to the under surface of the disc.The contact areas 15 and 17 are arcuate, covering a full circle on theupper surface of the disc except for an interruption substantiallybisecting the angle between the conductive strips 19 and 21. The disc 14having been prepared in the manner just mentioned is then placed in amask assembly comprising for example an outer annulus 23 together withan inner circular mask 24, centered and held by a connecting bar 25fitting into a locating slot in the annulus 23. A detailed descriptionof this type of mask and bolder assembly is found in the aforementionedconcurrently filed U.S. patent application of Lawrence B. Krauss. Inthis example the exposure area of the under side of the disc 14 is ringshaped except for a small angular area under bar 25. The disc 14, heldin the mask assembly just described, is positioned with its uncoatedarea facing downwardly upon a support 16 (Figure 1) having an opening topermit the vaporized coating material to condense upon the under face ofthe disc 14 and thus form the resistive coating in the desired arcuatepattern.

The material to be evaporated upon the disc 14 is deposited in acrucible 18 preferably of a high melting material such as tungsten,which can be heated by an electric current passed through the cruciblealong bars 20 which also support the crucible. At least one bar 20 mustbe insulated if the base is a conductor. The current leads for heatingthe crucible are shown at 22 and 24, and are passed through the base 10by suitable seals to preserve the vacuum.

To provide for the more or less continuous supply of small quantities ofthe basic mixture of the coating materials to the crucible 18 in theproper proportions to yield the desired composition in the finishedcoating, a small hopper 26 feeds the coating composition into aninclined tube 30 whose lower end is positioned substantially centrallyof the crucible 18 and a short distance above it. The hopper 26 iscarried by a bracket upon the framework of an electrical vibrator 32 (anordinary door buzzer may be used for this purpose) which in turn may besupported by any suitable means from the base plate 10. The electricalvibrator 32 is energized by an electrical circuit through leads 34 and36 extending through the base plate 10 by suitable vacuum seals. Thevoltage of the circuit for heating the crucible 18, and the voltage ofthe circuit for energizing the vibrator 32 may each be separatelycontrolled in a manner disclosed below. Thus the temperature of crucible18 may be controlled, as well as the actuation of the vibrator 32 toproduce more or less agitation of the hopper 26 to exercise control overthe rate at which coating material is fed into the crucible 18. i

To exercise close control over the coating operation it is necessarythat resistance measurements, either continuously or intermittently, bemade of the coating as it is being applied. To accomplish thesemeasurements there is provided an electrode structure 38 supported inany suitable manner within the bell jar 12. To facilitate the placingand removal of the disc 14 upon its support 16, the electrode structure38 should be supported in a manner to permit the desired movement. Suchan electrode structure support is described in detail in theaforementioned concurrently filed US. patent application of Krauss. Theelectrode structure 38 comprises a pair of contact pins 40 insulatedfrom each other, and so spaced that when the electrode structure isbrought to bear upon the upper face of the disc 14 as shown in Fig. 1,each electrode will contact one of the two contact areas and 17 (Fig.2). The contact pins 40 are connected by leads 42 and 44 through thebase 10 by suitable seals to a measurement and control system indicatedgenerally by the reference numeral 46.

A general description of the resistance and measurement control system46 will now be given in conjunction with the block diagram illustratedin Fig. 1. The input to the control circuit 46 is taken from the disc 14through contact pins 40 resting upon the conductive areas 15 and 17which in turn are connected by the conductive contact strips 19 and 21to the under side of the disc 14 where the arcuate strip resistivecoating is being applied. The contact pins 40 are connected at 43 and 45by leads 42 and 44 across a condenser 48 in one leg of a conventionalWheatstone bridge circuit designated generally by the reference numeral51). The bridge circuit 50 is sup plied with current from a source 52.The resistance introduced into the bridge circuit at 43 and 45 providesone leg of the bridge circuit. An adjacent leg, designated by thereference numeral 54, is of a predetermined resist 4 ance value, and ispreferably a decade resistance box which may be set to equal the desiredresistance of the coating to be applied to the disc 14. Equal. values ofresistors comprise the remaining two legs of the bridge in aconventional manner. The output of the bridge 51) depends on thedifierence in resistance between the resistive coating deposited on thedisc 14 and the standard resistance placed in the leg 54 of the bridge.As the resistive film is deposited on the disc 14 this difference inresistance becomes less and less until the bridge 50 is balanced,resulting in a zero output. If deposition of the resistive coating uponthe disc 14continues after the bridge is balanced, an error signaloutput is obtained with a change in polarity.

The direct current output error signal from the bridge 5%) is fedthrough lead 56 to a chopper 58 whereby it is converted into analternating current signal. The output of chopper 58 is fed to an AC.amplifier 66 to amplify the error signal taken from the bridge 50. Theoutput of the amplifier is fed to the grid 62 of a gas-filled controltube 64. A conventional thyratron may be used for this purpose. Theampllfier is designed so that its output will maintain a cut-01f bias onthe thyratron 64, preventing conduction until the bridge 50 is balanced,which condition will occur when the desired resistance value of thecoating on disc 14 is reached. When the bridge 50 reaches a state ofbalance, the thyratron control electrode voltage causes the tube 64 tofire. When control tube 64 becomes conductive, current from the anode 66flows to a relay 68 which causes the opening of the normally closedcontacts 70 and 72 in the heating circuit of the crucible 18 fed vialeads 22 and 24 from a suitable power source through an adjustableautotransformer 74. This halts any further evaporation of coatingmaterial still remaining in the crucible 18. Actuation of relay 68 bythe firing of the tube 64 also opens the normally closed contacts 76 and73 in the circuit for energizing the hopper vibrator 32 fed via leads34, 36 from a suitable power source through an adjustableautotransformer 80. This halts any further feed of coating material fromthe hopper 26 through tube 30 into the crucible 18.

Actuation of the relay 63 by the firing of control tube 64 serves toclose the normally open contacts 82 and 84 in the grid circuit ofthyratron 64, so that the grid- 62 is connected to ground to insureconduction until the circuit is reset for the next operation.

A delay relay 86 of the slow acting variety having contacts 88 and 9% issituated in the cathode-to-ground circuit of the thyratron 64 to preventchattering of the controlling relay 68 before the circuit is warmed up.A reset switch 92 is also provided in the grid circuit of the thyratron64 to enable that tube to regain control of the relay 68. This switchremoves the ground from grid 62 and, tube 64 being non-conducting,releases relay 68.

The following is a general description of the operation of the apparatusdescribed above. A blank disc 14 that has been properly prepared withconductive strips 19 and 21, is positioned on the mask 23 and the twoare placed upon the support 16 in the bell jar 12. The electrodestructure 38 is then positioned on the disc 14 so that contacts 46 arein firm contact with the conductive areas 15 and 17. The proper rawmaterial in a fine mixture to be vaporized is placed within the feedhopper 26, the bell jar 12 is secured upon the base 10 and the vacuumpumps are turned on to evacuate the space within the bell jar 12. Theevaporation control transformer 74 is adjusted so that the propertemperature will be attained by the crucible 18. At the same time thehopper vibrator control it is set to obtain the proper feed of coatingmaterial from the hopper 26 through feed tube 30 to the crucible 18 forevaporation. The resistance in the leg 54 of the Wheatstone bridge 50 isthen set to a value equal to that which it is desired to be coated uponthe disc 14. The resistance values in the other two legs of theWheatstone bridge are set to equal values.

The reset switch 92 is now momentarily closed, and the power applied tothe crucible heater, the hopper feed vibrator and the Wheatstone bridgethus placing the automatic control circuit 46 in control of the coatingoperation.

As the raw coating material is being initially vaporized from thecrucible 18 and being deposited upon the disc 14, the Wheatstone bridge50 is unbalanced and produces an output voltage which is converted intoan alternating current signal by the chopper 58, amplified by amplifier60 and applied as a bias to the grid 62 of the thyratron 64 to preventfiring of the tube. As evaporation continues the unbalance voltagedecreases until a point is reached when the bias on the thyratron fallsbelow the non-conduction point. When this occurs, the thyratron fires,operating relay 68 to open the vaporizer heating circuit and the hoppervibrator circuit as well, thus halting the vaporizing process. At thispoint the Wheatstone bridge 50 is balanced for the desired resistance onthe disc 14.

A specific embodiment of the circuit generally described in Fig. l andwhich has been found to operate satisfactorily is illustrated in detailin Fig. 3. In this embodiment, the control circuit contains aconventional power supply fed by a source of alternating current 100provided with a cut-off switch 102 and fuse. A power transformer 104connected to the alternating current source is provided with a filamentsupply secondary winding 106, a high voltage secondary winding 108, anda rectifier filament winding 110. A conventional full wave rectifiertube 112 as for example a 5Y3, is supplied its anode power fromtransformer secondary winding 108, and the cathode filament thereof issupplied from the transformer secondary winding 110 in the usual manner.A conventional filter network designated 114 is included in the outputof rectifier tube 112 for smoothing purposes.

Wheatstone bridge 50 is supplied necessary power for one diagonal fromthe transformer secondary winding 106 through a resistor 118 andrectifier 120. The resistance of the film being coated upon the disc 14is connected by contact pins 40 into the opposite diagonal of the bridgecircuit across condenser 48 in the measuring leg of the bridge. Anotherleg of the bridge is provided with accurate standard resistance 54 ofthe variable type (such as a decade box) which is capable of being setto the value of resistance equal to that desired to be coated upon thedisc 14, as described above. The remaining two legs of the bridge areeach provided with adjustable decade resistances 136, 138 having acommon control 142 so that each of these legs will have the same valueof resistance when varied. The purpose in providing adjustment of theselegs is to set the bridge for best sensitivity having regard to thedesired resistance of the deposit; e.g., whether in a low, medium orhigh range. Five range positions are shown, serving from 0 to 10 ohm to0 to 1,000,000 ohm ranges.

The output of the bridge is fed through the connection 56 to chopper 58to convert the direct current error voltage to an alternating currentsignal. The chopper may be any conventional type, one example being thecommercially available #243 Steven-Arnold chopper. A transformer 148connected directly across the main power source 180 supplies thenecessary power for operation of the chopper. A phase-controlpotentiometer 146 is incorporated into the chopper coil circuit toinsure the proper relationship between the output of the chopper and theanode voltage supply of the thyratron control tube 64.

The output of the chopper 58 is fed through a condenser 150 to thecontrol grid 152 of the first tube 154 of the two stage amplifier 60.Bias for the control grid 152 is obtained in the usual manner by meansof grid leak resistor 156. Convetnional pentode tubes, as for example,type 6817 may be utilized for both stages of the amplifier. The twostage amplifier is of the conventional resistance coupled type wellknown in the art. Two rectifiers and 162 which may be of the crystaldiode type are reversely connected to the output of the first amplifiertube 154; 160 from the B+ supply through dropping resistor 132 and 162from B-- via filter 163 to provide limiting of the output to the secondtube 164 of the amplifier when the error voltage from the Wheatstonebridge 50 is very large, as for example when the coating operation hasjust been initiated.

The output of the second amplifier tube 164 is coupled through condenser166, resistors 168 and 170 to the grid 62 of the gas filled control tube64, already described, which may be for example, the commerciallyavailable tube type 5696. The output from the second amplifier tube 164which is fed to the grid 62 of control tube 64 acts as a cut-off biasand prevents the tube 64 from firing as long as an error or unbalancevoltage is developed across the Wheatstone bridge 50 and is amplified bythe two stage amplifier. The output from the anode of control tube 64 isconnected to the winding of control relay 68 having two sets of normallyclosed contacts and one set of normally open contacts as alreadydescribed. As explained in connection with the arrangement in Figure l,the normally closed contacts complete the circuits to the crucibleheater, and to the coating feed hopper vibrator (not shown in Fig. 3).The normally open contacts, when closed, connect the control tube grid62 to ground. A reset switch 92 is provided in the control tube grid toground circuit to enable the grid 62 to gain control over the relay 68.

In the operation of this circuit, the standard adjustable resistance 54is set to the value which it is desired to obtain upon the disc 14. Thetwo resistances 138 and 136 are set to the desired range value by thecommon control 142, and the contacts 40 are placed properly upon thedisc 14 which is to be coated with a film of resistive material. Thepower switch 102 is turned on, and the reset switch 92 is closed. Thechamber being evacuated, and before any coating material is applied tothe disc 14, a very large unbalance or error voltage is developed acrossthe Wheatstone bridge 50. As the coating is applied to the disc 14 andthe resistivity of the coating increases, such error voltage willdecrease. This error voltage is converted into an alternating currentsignal by the chopper 58. The alternating current signal is amplified bythe two stage amplifier 60 and applied as a bias to the grid of thecontrol tube 64, to prevent firing of that tube. Control tube 64 remainsin a cut-off or non-conductive state until the error voltage across theWheatstone bridge is reduced to zero or changes polarity. At zero errorvoltage across the bridge (which is reached when the resistance value ofthe coating on disc 14 is of the proper value), the voltage appearing onthe grid of the control tube 64 permits the control tube to fire andactuate the relay 68. Actuation of the relay opens the vaporizer heatingcircuit, the feed hopper vibrator circuit, and connects the control tubegrid circuit to ground. Note that tube 64 is self-restoring to the cutoff condition because its anode is supplied with A.C. over lead 172.

To prevent chattering of the control relay 178 when the entire circuitis being warmed up for use, a slow acting relay 86 is placed in thecathode circuit of control tube 64in a conventional manner.

The circuit illustrated in Fig. 3 and described in detail above, hasbeen used with excellent success in controlling coating devices of thethermal vaporization type utilized in manufacturing precision roatrycontact potentiometer resistance elements. In tests made with thecontrol system described herein, it was possible to achieve an accuracyof better than 01% in seventy five percent of the resistorsmanufactured. The accuracy obtained by this control system as well asthe sensitivity and the time constant desired, depends in great measureupon the values of the resistances utilized in the Wheatstone bridge.The time constant of the system illustrated in Fig. 3 is roughly 0.1second, that is, the circuit will react and follow changes which takeplace in a minimum of 0.1 second. This time constant has been found tobe satisfactory for most purposes, but may be varied, by the use ofproper circuit components to any practical necessary value.

' As has been indicated above, while the measurement of attainedresistance in the manner indicated provides accurate information as tothe progress of the deposition procedure, at least two factors preventthe direct use of this measured resistance for controlling the cut-offpoint of the procedure. One factor is the tendency for vaporization ordeposition to continue for a small but appreciable interval after theheater circuit is de-energized; this is due in part to the thermal massof the crucible and other parts, and the presence of kinetic moleculesin the region between the crucible and the resistor itself.

For a given set of conditions, it is possible to correct for thisover-run by terminating the heater energization and/or material feed ashort time prior to perfect balance of the Wheatstone bridge. To thisend, an adjustable voltage divider 116 in the amplifier 60 may have itsadjustable contact connected to the control grid of the tube 164. Thesetting of this adjustable contact is readily arrived at after a littleexperience in the production of a particular precision resistor fromparticular materials, so that when the control relay 68 is energized,the normal over-run will carry the deposit very close to the desiredultimate value.

The second major factor influencing the accuracy of the final resistanceis the fact that the temperature of the resistance film duringdeposition is usually much higher than that at which the resistanceelement will be used. This correction can readily be made byartificially setting the comparison or standard resistance in theWheatstone bridge to a value slightly different from the ultimateresistance desired in the finished product. If the temperature atdeposition, and the thermal coeflicient of resistivity are known, thesetting of the comparison standard can readily be calculated; hereagain, experiment with a sample under actual production conditions willalso enable the operator to set the comparison resistance properly. Inaddition, this correction may be used to compensate for the known changein ultimate resistance which will be produced by any ageing orstabilization procedure which is to be applied to the resistor elementafter removal from the coating apparatus.

What is claimed is:

1. In a coating apparatus, a supply of resistive coating material,thermal vaporizing means, means for feeding small quantities of coatingmaterial to said thermal vaporizing means, means for supporting adielectric member having a pair of spaced conductive terminals fixedthereto, for exposure to the vaporized resistive material, means formeasuring the electrical resistance of the coating between saidterminals as it is being applied to said member, and means controlled bysaid measuring means to discontinue operation of said thermal vaporizerand said feed means when the electrical resistance of the coating onsaid member reaches a predetermined value.

2. In an apparatus for coating a dielectric member with electricallyresistive material, means for continuously applying the coating materialon a predetermined area of the member, means for making electricalcontact with the ends of the coating as it is being applied to saiddielectric member, a Wheatstone bnidge resistance measuring circuit, oneleg of said bridge including the resistance of the coating between saidcontact means, another leg of said bridge including a resistance of thevalue desired in said coating, control means connected to the 3 outputof said'bridge, said means being in a normally in active state when saidbridge is unbalanced, and said control means being activated uponbalance of said bridge to discontinue the application of the resistivematerial.

3. In a coating apparatus, means for continuously applying electricalresistance material to a dielectric article along a predetermined lengththereof, a Wheatstone bridge, one leg of said bridge including aresistance hav ing a value equal to the resistance desired on thearticle being coated, means electrically connecting the ends of thecoating on the article to another leg of said bridge, whereby saidbridge becomes balanced when the resistance value of said coatingbecomes equal to the resistance value in said first leg of the bridge,coating control means connected to the output of said bridge, and meansto operate said coating control means to terminate the coating actionwhen said bridge reaches a state of balance.

4. In a control system for a coating apparatus having means for applyingan electrical resistance coating upon an article of insulating materialalong a predetermined length thereof, a Wheatstone bridge, one leg ofsaid bridge including an electrical resistance having a value equal tothe desired resistance value of the article being coated, means toelectricallyconnect another leg of said bridge to the resistive coatingbeing applied to the article, means to amplify the output of saidbridge, control means connected to said amplifying means and normally inan inoperative condition when said bridge is unbalanced, and means toplace said control means in an operative condition to terminate thecoating action when said bridge is balanced.

5. In a control system for a coating apparatus having means for applyinga coating of electrical resistance material upon an article along apredetermined length thereof, a Wheatstone bridge, one leg of saidbridge including an electrical resistance having a value equal to thatdesired to be coated upon the article, means for introducing to anotherleg of said bridge the resistance of the article as it is being coated,means to automatically balance said bridge when a desired resistancevalue of the coating is reached, control means for said apparatusconnected tothe output of said bridge, said control means including agas switching tube normally in a cut-off condition, and means to placesaid tube in a conductive condition to terminate the coating operationwhen said bridge is balanced.

6. In a control system for a coating apparatus having means for applyinga coating of electrical resistance material upon an article ofinsulating material along a predetermined length thereof, a Wheatstonebridge, one leg of said bridge containing an electrical resistance ofthe same value that is desired to be coated upon the article, means forelectrically connecting to another leg of said bridge the resistance ofthe article being coated, means connected to the output of said bridgeto convert any unbalance into an alternating current signal, means toamplify said signal, control means connected to the output of saidamplifier, said control means including an electron tube normally biasedto cut-off when said bridge is unbalanced, and means including saidbridge connected to remove said bias when the bridge is balanced wherebysaid electron tube vecomes conductive to terminate the coating action.

7. In a control system for a coating apparatus having means for applyingan electrical resistance coating upon an article of insulating materialalong a predetermined length thereof, an electrical resistance having apredetermined value, means for electrically comparing said resistance tothe resistance value of the coating being applied to the article, meansto amplify the difference be tween said values, control means connectedto said comparing means and normally in an inoperative condition whensaid resistances are unequal, means to place said control means in anoperative condition when said coating attains a resistance equal to saidpredetermined resistance, and means under the control of said controlmeans for terminating the coating operation.

8. In a control system for a coating apparatus having means for applyingan electrical resistance coating upon an article of insulating materialalong a predetermined length thereof, a Wheatstone bridge, one leg ofsaid bridge including an electrical resistance having a value equal tothe desired resistance value of an article being coated, means toelectrically connect another leg of said bridge to the resistive coatingbeing applied to the article, means to amplify the output of saidbridge, control means connected to said amplifying means and normally inan inoperative condition when said bridge is unbalanced, means to placesaid control means in an operative condition when said bridge isbalanced, means for adjusting the operating point of said control meanswith reference to the balance point of said bridge, and means under thecontrol of said control means for terminating the coating operation.

9. A process for the production of a resistor having a predeterminedvalue, comprising applying a coating of electrical resistance materialto a dielectric member between two spaced terminals thereon, graduallyincreasing the thickness of the coating by continuously applyingadditional such material thereto, continuously measuring the resistanceof the resulting coating between the terminals during the coatingoperation, and terminating the further application of resistive coatingmaterial when a predetermined value of resistance is reached.

References Jilted in the file of this patent UNITED STATES PATENTS2,153,786 Alexander et al. Apr. 11, 1939 2,239,452 Williams et al Apr.22, 1941 2,273,941 Dorn Feb. 24, 1942 2,338,234 Dimmick Jan. 4, 19442,482,196 Marye Sept. 20, 1949 2,545,576 Godley Mar. 20, 1951 2,635,225Hadady Apr. 14, 1953

9. A PROCESS FOR THE PRODUCTION OF A RESISTOR HAVING A PREDETERMINEDVALUE, COMPRISING APPLYING A COATING OF ELECTRICAL RESISTANCE MATERIALTO A DIELECTRIC MEMBER BETWEEN TWO SPACED TERMINALS THEREON, GRADUALLYINCREASING THE THICKNESS OF THE COATING BY CONTINUOUSLY APPLYINGADDITIONAL SUCH MATERIAL THERETO, CONTINUOUSLY MEASURING THE RESISTANCEOF THE RESULTING COATING BETWEEN THE TERMINALS DURING THE COATINGOPERATION, AND TERMINATING THE